Amorphous Salts in the Martian Regolith.

Orbital and landed instruments at Mars have provided significant advances in our understanding of Martian sediments, as well as a number of new questions. For example, the Mars Science Laboratory (MSL) CheMin X-ray diffractometer instrument has detected x-ray amorphous (hereafter, “amorphous”) materials in every sediment and rock measurement thus far, with abundance estimates ranging from ~30-45 wt%. Supporting measurements from the APXS and SAM instruments have constrained the bulk chemistry (including S and Cl) and hydrated nature of the amorphous fraction; however, in general, the phase(s) that comprise this fraction are poorly constrained. A variety or combination of phases are plausible, including volcanic/impact glasses, allophane, hisingerite, amorphous sulfate and other salts, or other nanophases.

     Here, we explore the possible role of S-bearing amorphous salts in contributing to the amorphous component of sediments measured by MSL. Based on previous experiments, a plausible formation pathway for amorphous sulfate is via rapid dehydration of a concentrated ferric sulfate brine. Brines could form through deliquescence of crystalline ferric sulfates at the grain scale, under conditions of high relative humidity (RH) in the Martian shallow subsurface. While deliquesced, the brines form a sticky, amber-colored gel; this gel solidifies into an amber-colored glass upon rapid dehydration. Once amorphized, these materials are relatively resistant to moderate RH fluctuations, suggesting that they could persist for long periods of time on the Martian surface. Furthermore, preliminary experiments suggest that addition of other elements to the brine, such as Na, can cause the amorphous product to be even more resistant to RH fluctuations. We conclude that amorphous S-bearing salts are plausible, if not expected, components of Martian regolith. If correct, there are several implications with regards to sample return, in-situ resource utilization, biochemical availability/energy sources, and mission hardware environments. These will be discussed at the conference.